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How Should We Prioritize Which Exoplanets We Probe For Signs Of Life?

Although “habitable” planets, such as Kepler-186f, have dominated our search for alien life, this will have to be narrowed down to planets where life could be definitively “detected” to help direct future missions. NASA Exoplanet Exploration

There has been a lot of talk recently about how we can best utilize future observations from the next generation of ground and space-based telescopes in our search for alien life. Indeed, as the much-delayed James Webb Space Telescope is predicted to characterize only five exoplanet atmospheres in its mission lifetime, it is necessary to find a way to prioritize planets whose data could definitively yield an answer as to whether life exists there or not.

To create one such framework, an interdisciplinary team of researchers, led by Arizona State University (ASU), took a closer look at one potential sign of life (biosignature): oxygen. Whilst on Earth this gas is produced by biological processes, on other planets, the abundance of oxygen could be purely attributed to non-biological processes. In order to distinguish those planets where a detection of the gas conclusively reveals the presence of life, the team created a “detectability index.”


“The goal of the index is to provide scientists with a tool to select the very best targets for observation and to maximize the chances of detecting life,” Donald Glaser, lead author of the study published in The Astrophysical Journal, said in a statement.

Earth’s oxygen detectability index would be high, as the oxygen in its atmosphere is definitely down to life. However, the team discovered that the index falls dramatically for other Earth-like planets, where water accounts for a larger percentage of its mass; oceans only make up 0.025 percent of Earth’s total mass.

“It’s easy to imagine that in another Solar System like ours, an Earth-like planet could be just 0.2 percent water,” co-author Steven Desch of ASU’s School of Earth and Space Exploration, explained. “And that would be enough to change the detectability index. Oxygen would not be indicative of life on such planets, even if it were observed. That’s because an Earth-like planet that was 0.2 percent water — about eight times what Earth has — would have no exposed continents or land.”

With no land, it would not be possible for rain to weather rocks and free key nutrients (such as phosphorous) that enhance the oxygen-producing biosphere (areas of life) in the oceans. Therefore oxygen from photosynthetic life would be released at a much lower rate and become indistinguishable to the oxygen produced non-biologically.

Without nutrient runoff, the oxygen produced by the biosphere in a planet with a higher mass of water is significantly reduced. D. Glaser/ASU

So for oxygen to be a reliable biosignature (ie a sure-fire marker of life), the researchers suggest that both water and land need to be directly detected as well. But with potentially only five exoplanets to probe, the study further recommends that planets with a water content greater than 0.2 percent are de-prioritized, “at least in searches for life on exoplanets using atmospheric oxygen.” This will help future missions pick the best targets to look for life on.

“The detection of life on a planet outside our Solar System would change our entire understanding of our place in the universe,” Glaser said. “NASA is deeply invested in searching for life, and it is our hope that this work will be used to maximize the chance of detecting life when we look for it.”


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